why don't you make that a fixture?

@pytest.fixture
def cli_client(mocker):
    mock_client = MagicMock()
    mock_client.service_document.return_value = EXAMPLE_SERVICE_DOCUMENT
    mock_client.deposit_create.return_value = '{"foo": "bar"}'
    mocker.patch(
        'swh.deposit.cli.client._client',
        return_value=mock_client)
    return mock_client

That will unclutter a bit the scenario below.
If that makes sense.

Note:
you can use the factory pattern if you need some different output in the deposit.create.return_value mock.

that's interesting! [1]

[1] https://docs.pytest.org/en/latest/logging.html#caplog-fixture

jsyk, as @moranegg probably already told you this is kind of a pain in deposit.

The slug is a word used in the sword v2 protocol (it makes no sense to me).
It's needed because it's required in the spec.
Thus, the uuid generation somewhere in the cli here when it's not provided (it needs to be unique).

We decided to use this as external_id, so whenever you see that, think external_id as a unique identifier for the client system...

I don't like the fact that we use both terms but i was a bit worried to unify it under what term and hide that somewhere at the time. Maybe it'd make more sense to rename it once and for all though (later, not right now ;).